Method of purifying nanodiamond powder and purified nanodiamond powder

ABSTRACT

A method of purifying a nanodiamond powder includes preparing the nanodiamond powder, heating the nanodiamond powder at between 450° C. and 470° C. in an atmosphere including oxygen, performing a hydrochloric acid treatment on the heated nanodiamond powder, and performing a hydrofluoric acid treatment on the nanodiamond powder obtained after performing the hydrochloric acid treatment.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.13/536,502 filed Jun. 28, 2012, which is hereby incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of purifying nanodiamondpowder and highly-purified nanodiamond powder.

2. Description of the Related Art

Nanodiamonds are used as abradants because of their hardness, and theycan also be used as an insulating material, an optical material, and abiomedical material.

In WO2007/133765, Gogotsi, et al. disclosed an oxidization process toremove sp² carbon from commercially available nanodiamond powder.However, a small percentage of sp² carbon can still be detected afterthe oxidization process. In addition to the removal of sp² carbon, metalimpurities might be an issue when the nanodiamonds are used as anoptical or biomedical material.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide an apparatus and a methodfor nanodiamond purification.

According to an aspect of the present invention, a purification methodof a nanodiamond powder is provided that includes preparing thenanodiamond powder, heating the nanodiamond powder at between 450° C.and 470° C. in an atmosphere including oxygen, performing a hydrochloricacid treatment to the heated nanodiamond powder, and performing ahydrofluoric acid treatment to the nanodiamond powder obtained afterperforming the hydrochloric acid treatment.

According to another aspect of the present invention, a nanodiamondpowder is provided having sp³ carbon, wherein the content of S (sulfur),Fe (iron), Al (aluminum), and Si (silicon) in the powder is each lessthan 0.01 wt %.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a thermogravimetric analysis (TGA) spectrum of ananodiamond powder before oxidation.

FIGS. 2A and 2B show electron energy loss spectroscopy (EELS) spectra ofa nanodiamond powder before and after heat treatment, respectively.

FIGS. 3A, 3B, 3C, and 3D show energy dispersive x-ray spectroscopy(EDAX) spectra of a nanodiamond powder at different steps in apurification process, and each curve shows a different measurement pointin the same sample.

FIG. 4 shows EELS spectra of a nanodiamond powder at different steps ina purification process.

FIG. 5 shows EELS maps recorded on nanodiamond powder after heattreatment.

FIG. 6A shows blue and gray nanodiamond powders separated usingcentrifugation.

FIG. 6B shows white, blue and gray nanodiamond powders separated using adensity gradient separation technique, along with their transmissionelectron microscopy (TEM) images.

DESCRIPTION OF THE EMBODIMENTS

According to aspects of the invention, a purification method forpurifying nanodiamond powder can include the following three steps. Thefirst step is a heating step to oxidize carbon impurities (sp² carbon)in order to remove them from a nanodiamond powder. The heating can beexecuted in an atmosphere including oxygen. The atmosphere can be anair, or an oxygen gas. The temperature of the heating can be selectedfrom between 450° C. and 470° C. For example, the powder can be heatedat 460° C. FIG. 1 shows the TGA spectrum of as-purchased nanodiamondpowder. The oxidation process onsets at about 460° C. (indicated as adashed line) in this spectrum, as evident from the radiated thermalenergy peak.

When a purchased nanodiamond powder substantially doesn't have sp²carbon before the first step, the heating step can be omitted.

The second step is to remove metal impurities, especially transitionmetals, from the nanodiamond powder after the oxidization process. Toremove the metal impurities, a hydrochloric acid can be used.Hydrochloric acid may provide a high reaction rate with the transitionmetals. The hydrochloric acid may be provided as a solution of from 10%to 38.5% by weight of hydrochloric acid, such as a solution of 37% byweight hydrochloric acid. Hydrobromic and hydroiodic acids might also beused instead of using the hydrochloric acid.

The third step is to remove other impurities, such as silicon, silicondioxide, aluminum, and aluminum silicate. To execute the third step, ahydrofluoric acid can be used. The hydrofluoric acid may be provided asa solution of from 4% to 48% by weight of hydrofluoric acid, such as asolution of 47% by weight hydrofluoric acid.

After the third step, a density gradient technique can be executed inorder to classify the nanodiamonds, optionally. The density gradienttechnique allows for separation of the purified nanodiamonds on thebasis of density, which may provide separation of nanodiamond materialsaccording to particle size and/or number of particle clusters. Accordingto one embodiment of a density gradient technique, a linear densitygradient can be prepared using 60% OptiPrep™ (a commercially availabledensity gradient solution comprising a solution of iodixanol) indistilled water, with relative densities of 60%, 50%, 40%, 30%, and 20%.One beaker is prepared for each ratio and stirred for 15 minutes toensure good mixing. 14 ml of each in order of decreasing density arecarefully added to a 70 ml centrifuge tube via syringe. The resultingstructure is centrifuged at 3500 rpm for 20 minutes in order tolinearize the density within the tube. A sample of centrifugednanodiamond, possessing both a blue layer and a gray bulk material, isagitated in deionized water and 1 gram sodium cholate surfactant. Themixture is then sonicated and injected into the linear density gradientwhich is centrifuged at 3500 rpm for 99 minutes. The small amount ofmaterial spreads out to form a large cloud, and liquid is extracted fromthree regions within the density gradient and centrifuged separately.The isolated solid is collected and dissolved in a small amount ofdeionized water. Each of the solids collected from the extracted regionsof the density gradient can be further classified according to one ormore of color, particle size and particle cluster size, among otherfeatures, as discussed in further detail below.

Example 1

Nanodiamond powder can be produced via a detonation synthesis, and alsocommercially obtained.

Step I: Isothermal Treatment

An example of an isothermal treatment procedure is as follows.As-purchased nanodiamond powder (1.57 g) was placed in an oven at 460°C. for 1 hr. After removing it from the oven, the resulting powder wascooled down to room temperature to give ND_Iso460-1hr as a light graypowder (1.43 g, 91% yield).

Isothermal cleaning at 460° C. for approximately 1 hr effectivelyremoves sp² carbon that is present in the purchased powder. The level ofsp² carbon has primarily been estimated by electron energy lossspectroscopy (EELS), where spectral features are observed associatedwith sp² carbon (at ˜285 eV) and sp³ carbon (at about ˜294 eV). The EELSspectra can be recorded using a scanning transmission electronmicroscope (JEOL 2010F), operated at 200 kV, equipped with TEM/STEM andGatan high-resolution GIFT EELS detectors.

As an example, FIGS. 2A and 2B show the EELS spectra for the nanodiamondpowder, before and after the isothermal treatment, respectively.According to FIGS. 2A and 2B, it is observed that sp² carbonsubstantially doesn't exist in the powder after isothermally heating thesp² carbon. When a purchased nanodiamond powder substantially doesn'thave sp² carbon before the isothermal cleaning step, this step can beomitted.

After the isothermal treatment, the content of sp² carbon was found tobe less than 0.01 wt %, as determined from high resolution EELS mapsobtained after isothermal treatment.

FIG. 5 shows EELS maps recorded on isothermally cleaned nanodiamondpowder. Using GATAN EELS analysis software the concentration of sp³ Cwas calculated as 2042±125 atoms/nm³, which corresponds to 3.39±0.21g/cm³, comparable to diamond density of 3.515 g/cm³. Similarmeasurements over the reconstructed sp² map over 284-290 eV yields a sp²concentration 4.6×10⁻⁴ atoms/nm² with residual map density of ˜10⁻³atoms/nm². Assuming the residual map density as the measurement noiselevel, it is concluded that the sp² C/sp³ C ratio is ˜5×10⁻⁷, whichcorresponds to less than 0.01 wt % of sp² carbon.

Step II: Hydrochloric Acid Treatment

An example of an HCl treatment procedure is as follows. The nanodiamondpowder (1.21 g) obtained after the isothermal treatment was placed in 30mL of 37% hydrochloric acid. The mixture was heated to 120° C. withstirring for 1 hr. After heating at 120° C. for 1 hr, the resultingsuspension was poured into 200 mL of deionized water. The suspension wasallowed to settle overnight to precipitate sufficient nanodiamond. Thesupernatant was removed gently. The remaining precipitate was repeatedlyrinsed with deionized water until reaching the same pH as deionizedwater, and dried overnight at 100° C. in vacuo to giveND_Iso460-1hr_HCl-1 hr as a light gray powder (0.91 g, 75% yield).

Step III: Hydrofluoric Acid Treatment

An example of an HF cleaning treatment procedure was performed asfollows. Nanodiamond powder (0.20 g) yielded from the above HCltreatment was placed in 10 mL of 47-51% hydrofluoric acid. The mixturewas stirred at room temperature for 80 min before the resultingsuspension was poured into 200 mL of deionized water. This suspensionwas allowed to settle for 3 hours to precipitate sufficient nanodiamond.The supernatant was removed gently. The remaining precipitate wasrepeatedly rinsed vigorously with deionized water until reaching thesame pH as deionized water, and dried overnight at 100° C. in vacuo togive ND_Iso460-1 hr_HCl-1hr_HF-80 min as a light gray powder (0.15 g,75% yield). Plastic labware was used here instead of laboratoryglassware, due to the corrosive effects of HF.

Energy dispersive X-ray (EDAX) analysis is a technique that can beperformed with a scanning electron microscope and can providequantitative information on the elements present in a given field ofview. For each sample, eleven fields (50 μm×50 μm) were analyzed. TheEDAX spectra were observed with the Hitachi S-3400 and S-4800. K_(α)*energies between 0 and 11 keV were measured, which covers the majorityof elements in the periodic table combining Lα and kα peaks.

FIG. 3A shows EDAX spectra of nanodiamonds as-purchased. The horizontalline is energy in kiloelectron volts (keV), and the vertical line (notshown) is relative intensity.

The primary peak positions of S (sulfur), W (tungsten), Ta (tantalum),Fe (iron), Cr (chromium), Mn (manganese), Al (aluminum), Ag (silver), Ca(calcium), Cu (copper), Ti (titanium), Si (silicon), and Cl (chlorine)are 2.307 (S), 1.774 and 8.396 (W), 1.709 and 8.145 (Ta), 0.705 and6.403 (Fe), 0.573 and 5.414 (Cr), 0.637 and 5.898 (Mn), 1.486 (Al),2.984 (Ag), 0.341 and 3.691 (Ca), 0.930 and 8.040 (Cu), 0.452 and 4.510(Ti), 1.740 (Si), 2.622 (Cl), respectively.

FIGS. 3B through 3D show nanodiamond EDAX spectra after the each of thepurification steps, namely isothermal treatment, hydrochloric acidtreatment, and hydrofluoric acid treatment.

FIG. 3B shows the results after isothermal cleaning. There are onlyminor changes in the EDAX spectrum after isothermal cleaning, since nometals are presumed to be removed by this process.

The hydrochloric acid step (second step) removes transition metals suchas Fe, Cr, Ni, as well as other metals that readily form a water solublemetal chloride. FIG. 3C shows EDAX spectra after HCl cleaning, where itcan be clearly seen that many of the metals present in the as-receivedand isothermally cleaned samples in FIGS. 3A and 3B are no longerpresent, in fact, almost all that remains are Al, Si, S, Cl and smallamounts of iron.

In the hydrofluoric acid treatment (third step) of the cleaning process,the remaining non-carbon elements, primarily Al and Si, are removedusing hydrofluoric acid. FIG. 3D shows the EDAX spectra after the HFcleaning step, and it can be seen that only carbon and oxygen are leftas impurities in the sample.

The above described three-step purification process was designed toremove the carbon impurities (non-sp³ carbon) and inorganic impurities(metal species, oxides, etc.) which are included in commerciallyavailable nanodiamonds. The purification process may include (1)isothermal treatment for removal of non-diamond carbon, (2) hydrochloricacid treatment for removal of transition metals, and (3) hydrofluoricacid treatment for removal of remaining non-carbon species, includingaluminum, and silicon.

Table 1 summarizes the quantitative EDAX data showing the wt % ofspecies in the nanodiamond powder during different stages of thepurifying process, including for the powder as-received, afterisothermal heat treatment, after hydrochloric acid treatment, and afterhydrofluoric acid treatment. The table shows that the completely cleanednanodiamond powder substantially does not have any found metals (NF=notfound), at the level of 0.01 wt %. The level of 0.01 wt % is arrived atusing a high-resolution leading edge EDAX measurement system such as inS-4800 and JEOL 2010F, which has resolution (lowest detectionsensitivity) of few atomic %, particularly for elements with atomicnumber Z>11. Sensitivity of this technique increases with increasingatomic number. Accordingly, as an array of elements with varied atomicnumbers are included, the detection limit (lowest detection sensitivity)can be conservatively estimated as 0.01 wt %. This data, together withthe EELS data, shows that the major extrinsic sources of color, namelysp² carbon and metal impurities, have been removed from the nanodiamondto at least the level of 0.01%, and significantly lower in the case ofsp² carbon.

TABLE 1 Quantitative EDAX data (unit: wt %) Isothermal As 460° C., HClHF Component Received 1 hr, Air treatment treatment S 1.17-1.35 0.8-0.90.75-0.9  NF W 0.25 0.24 NF NF Ta 0.18 0.19-0.2  NF NF Fe 0.1  0.15-0.2 <0.01 NF Cr 0.3-0.5 0.5-0.7 NF NF Mn 0.01 0.02 NF NF Al 0.01 0.08 0.1-0.12 NF Ag 0.16 <0.01  NF NF Ca 0.01 0.01-0.02 NF NF Cu 0.05 0.04NF NF Ti 0.02 0.04 NF NF Si 0.15  0.2-0.25  0.2-0.35 NF Cl 1.1-1.21.2-1.3  0.05 NF

Example 2

In order to establish that the improved cleaning process does notreintroduce sp² carbon, a series of EELS measurements were made onnanoparticles taken after each step by using a commercial nanodiamondpowder with little or no sp² C, purchased from Sigma-Aldrich. FIG. 4shows that, other than overall signal level variation, there is nochange in the EELS spectrum during the cleaning process. In FIG. 4, thehorizontal line is energy in eV, and the vertical line (not shown) isEEL signal count.

After HF cleaning and washing with water, the nanodiamond powder wassubjected to centrifugation, upon which the powder separated into twodistinct layers, blue on top of gray, as shown in FIG. 6A. Undertransmission electron microscopy (TEM) the blue layer was found tocontain smaller particles with the least number of particle clusters.The gray was found to contain mostly clusters of particles. Sincenanoparticle clusters often exhibit increased optical scattering, theblue layer having fewer clusters is of interest for opticalapplications, such as in developing high refractive index composites.Purified gray material can also be used for other applications such assensors or abrasive materials. The blue layer contained smallerparticles but also a large range of diameters. To further size purifythese particles the blue layer was subjected to the density gradientseparation technique, as described above.

Example 3

By using the density gradient technique described above, the nanodiamondpowder can be classified as follows.

FIG. 6B shows the separated white, blue and gray nanodiamond powdersalong with their transmission electron microscopy (TEM) images. Thewhite layer (collected from the top in the density gradient method)contained particles with diameter 7±2 nm, and clusters with averagediameter of 35±10.2 nm. The blue layer (collected from the bottom layer)contained particles with diameter of 5±4 nm and clusters of 102±60.0 nm.The gray layer, which is very similar to bulk, was found to containparticles with diameter of 5±7 nm and 220±85.0 nm clusters. Thus, thedensity gradient separation technique allows for the separation ofnanodiamond layers according to particle sizes and/or number of particleclusters, such as to allow for isolation of layers having reducedparticle sizes and/or reduced numbers of particle clusters. Of theseparated layers, the nanodiamond particles recovered from the whitelayer may provide characteristics suitable for optical applicationsbecause of low optical scattering from the smaller particles and thesmall number of clusters present in the white layer. Particles from theblue layer can also be used for wavelength specific optical applicationssuch as luminescent particles in biological tagging and as sensors. Theparticles separated from the gray region are very similar to the onesseparated from the gray layer following centrifugation as describedabove, and can be used for similar applications.

A cluster was defined as a collection of individual nanoparticles thatare attached to each other in the TEM images, a minimum number ofparticles that were considered in this estimation of cluster size was 2.TEM images recorded in 300 k magnification were used and at thismagnification TEM instrument has ˜0.6 nm resolution. Using programImageJ the pixels were calibrated for the size based on the sale barprinted on each TEM image. A total of 5 sets of TEM images were used foreach particle.

Nanodiamonds have been widely used in applications ranging amongabrasives, coatings, lubricants, polymer composites, electronic devices,and the biomedical field. Nanodiamonds cleaned according to aspects ofthe invention can work more effectively for these and other applicationsthan commercial nanodiamonds containing some impurities. Aspects of theinvention can provide a high efficiency purification method, as shownabove, which yields nanodiamonds that are much purer than nanodiamondsproduced by existing cleaning processes.

Despite the removal of sp² carbon and substantially all measurable metalimpurities in the cleaning process, the resulting nanodiamond may remaina light gray color in some cases. Accordingly, further separation oflarge particles using the disclosed density gradient technique has beenshown to be effective in further reducing gray color.

As explained above, sp² carbon and metal contaminations can be removedto obtain purified nanodiamonds. The purified nanodiamonds can fullydemonstrate their bulk character with respect to mechanical properties,chemical stability, optical properties (i.e. high refractive index),electric properties, and thermal conductivity. They can be used asmaterials for transparent resin, glass, or plastic. For example, thepurified nanodiamonds can be used in a glass lens or a plastic lens.Since the content of the metal impurities in the purified nanodiamondsis very low, such purified nanodiamonds can also be used as biomedicalmaterials.

While the embodiments according to the present invention have beendescribed with reference to exemplary embodiments, it is to beunderstood that the present invention is not limited to the abovedescribed embodiments. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

What is claimed is:
 1. Nanodiamond powder comprising sp³ carbon, whereina content of sp² carbon is less than 0.01 wt %; and a content of S(sulfur), Fe (iron), Al (aluminum) and Si (silicon) is each less than0.01 wt %, wherein the nanodiamond powder comprises clusters of ananodiamond particle, and an average diameter of the cluster is lessthan 102 nm.
 2. The nanodiamond powder according to claim 1, wherein acontent of W (tungsten), Ta (tantalum), Cr (chromium), Mn (manganese),Ag (silver), Ca (calcium), Cu (copper), and Ti (titanium) is each lessthan 0.01 wt %.
 3. The nanodiamond powder according to claim 2, whereinthe content of W (tungsten), Ta (tantalum), Cr (chromium), Mn(manganese), Ag (silver), Ca (calcium), Cu (copper), and Ti (titanium)is each determined by EDAX.
 4. The nanodiamond powder according to claim1, wherein carbon and oxygen are present; and S (sulfur), W (tungsten),Ta (tantalum), Fe (iron), Cr (chromium), Mn (manganese), Al (aluminum),Ag (silver), Ca (calcium), Cu (copper), Ti (titanium), and Si (silicon)are not substantially present, wherein presence is determined by EDAX.5. The nanodiamond powder according to claim 4, wherein the content of S(sulfur), W (tungsten), Ta (tantalum), Fe (iron), Cr (chromium), Mn(manganese), Al (aluminum), Ag (silver), Ca (calcium), Cu (copper), Ti(titanium), and Si (silicon) is each determined by EDAX.
 6. Thenanodiamond powder according to claim 1, wherein the content of S(sulfur), Fe (iron), Al (aluminum) and Si (silicon) is each determinedby EDAX.
 7. The nanodiamond powder according to claim 1, wherein thecontent of sp² carbon is determined by EELS.
 8. The nanodiamond powderaccording to claim 1, wherein an average diameter of particles is lessthan 10 nm.
 9. The nanodiamond powder according to claim 1, wherein acolor of the powder is white or colorless.
 10. The nanodiamond powderaccording to claim 1, wherein a color of the powder is blue.
 11. Thenanodiamond powder according to claim 1, obtained by a processcomprising: preparing the nanodiamond powder; heating the nanodiamondpowder at between 450° C. and 470° C. in an atmosphere including oxygento reduce a content of sp² carbon; after heating the nanodiamond powderto reduce the content of sp² carbon, performing a hydrochloric acidtreatment on the heated nanodiamond powder; and performing ahydrofluoric acid treatment on the nanodiamond powder obtained after thehydrochloric acid treatment, wherein a content of S (sulfur), Fe (iron),Al (aluminum) and Si (silicon) is each reduced.
 12. The nanodiamondpowder according to claim 1, obtained by a process comprising:performing a density gradient separation process to separate thenanodiamond powder according to one or more of particle size and numberof particle clusters.
 13. Nanodiamond powder comprising clusters of ananodiamond particle comprising sp³ carbon, wherein an average diameterof clusters is less than 45.2 nm; a content of sp² carbon is less than0.01 wt %; and a content of S (sulfur), Fe (iron), Al (aluminum) and Si(silicon) is each less than 0.01 wt %.
 14. The nanodiamond powderaccording to claim 13, wherein an average diameter of particles is lessthan 10 nm.
 15. The nanodiamond powder according to claim 13, wherein acolor of the powder is white or colorless.
 16. The nanodiamond powderaccording to claim 13, wherein a color of the powder is blue.
 17. Thenanodiamond powder according to claim 13, wherein a content of W(tungsten), Ta (tantalum), Cr (chromium), Mn (manganese), Ag (silver),Ca (calcium), Cu (copper), and Ti (titanium) is each less than 0.01 wt%.
 18. The nanodiamond powder according to claim 17, wherein the contentof W (tungsten), Ta (tantalum), Cr (chromium), Mn (manganese), Ag(silver), Ca (calcium), Cu (copper), and Ti (titanium) is eachdetermined by EDAX.
 19. The nanodiamond powder according to claim 13,wherein carbon and oxygen are present; and S (sulfur), W (tungsten), Ta(tantalum), Fe (iron), Cr (chromium), Mn (manganese), Al (aluminum), Ag(silver), Ca (calcium), Cu (copper), Ti (titanium), and Si (silicon) arenot substantially present, wherein presence is determined by EDAX. 20.The nanodiamond powder according to claim 19, wherein the content of S(sulfur), W (tungsten), Ta (tantalum), Fe (iron), Cr (chromium), Mn(manganese), Al (aluminum), Ag (silver), Ca (calcium), Cu (copper), Ti(titanium), and Si (silicon) is each determined by EDAX.
 21. Thenanodiamond powder according to claim 13, wherein the content of S(sulfur), Fe (iron), Al (aluminum) and Si (silicon) is each determinedby EDAX.
 22. The nanodiamond powder according to claim 13, wherein thecontent of sp² carbon is determined by EELS.
 23. The nanodiamond powderaccording to claim 13, obtained by a process comprising: preparing thenanodiamond powder; heating the nanodiamond powder at between 450° C.and 470° C. in an atmosphere including oxygen to reduce a content of sp²carbon; after heating the nanodiamond powder to reduce the content ofsp² carbon, performing a hydrochloric acid treatment on the heatednanodiamond powder; and performing a hydrofluoric acid treatment on thenanodiamond powder obtained after the hydrochloric acid treatment,wherein a content of S (sulfur), Fe (iron), Al (aluminum) and Si(silicon) is each reduced.
 24. The nanodiamond powder according to claim13, obtained by a process comprising: performing a density gradientseparation process to separate the nanodiamond powder according to oneor more of particle size and number of particle clusters.